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[MemorySSA] Add previous def to cache when found, even if trivial. 

Summary:
When inserting a new Def, MemorySSA may be have non-minimal number of Phis.
While inserting, the walk to find the previous definition may cleanup minimal Phis.
When the last definition is trivial to obtain, we do not cache it.

It is possible while getting the previous definition for a Def to get two different answers:
- one that was straight-forward to find when walking the first path (a trivial phi in this case), and
- another that follows a cleanup of the trivial phi, it determines it may need additional Phi nodes, it inserts them and returns a new phi in the same position as the former trivial one.
While the Phis added for the second path are all redundant, they are not complete (the walk is only done upwards), and they are not properly cleaned up afterwards.

A way to fix this problem is to cache the straight-forward answer we got on the first walk.
The caching is only kept for the duration of a getPreviousDef call, and for Phis we use TrackingVH, so removing the trivial phi will lead to replacing it with the next dominating phi in the cache.
Resolves PR40749.

Reviewers: george.burgess.iv

Subscribers: jlebar, Prazek, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D60634

llvm-svn: 358313
This commit is contained in:
Alina Sbirlea 2019-04-12 21:58:52 +00:00
parent 2806fd01a1
commit f9f073a861
2 changed files with 65 additions and 1 deletions
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4
llvm/lib/Analysis/MemorySSAUpdater.cpp
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@ -156,8 +156,10 @@ MemoryAccess *MemorySSAUpdater::getPreviousDefFromEnd(
DenseMap<BasicBlock *, TrackingVH<MemoryAccess>> &CachedPreviousDef) {
auto *Defs = MSSA->getWritableBlockDefs(BB);
if (Defs)
if (Defs) {
CachedPreviousDef.insert({BB, &*Defs->rbegin()});
return &*Defs->rbegin();
}
return getPreviousDefRecursive(BB, CachedPreviousDef);
}

62
llvm/test/Analysis/MemorySSA/pr40749_2.ll Normal file
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@ -0,0 +1,62 @@
; RUN: opt -S -licm -loop-unswitch -enable-mssa-loop-dependency -verify-memoryssa %s | FileCheck %s
; REQUIRES: asserts
target datalayout = "E-m:e-i1:8:16-i8:8:16-i64:64-f128:64-v128:64-a:8:16-n32:64"
target triple = "s390x-ibm-linux"
@g_92 = external dso_local local_unnamed_addr global i16, align 2
@g_993 = external dso_local local_unnamed_addr global i32, align 4
; CHECK-LABEL: @ff6
define dso_local fastcc void @ff6(i16 %arg1) unnamed_addr #0 {
bb:
%tmp6.i = icmp sgt i16 %arg1, 0
br label %bb10
bb10: ; preds = %bb81.loopexit, %bb
%tmp17 = load i16, i16* @g_92, align 2
%tmp18 = add i16 %tmp17, 1
store i16 %tmp18, i16* @g_92, align 2
br label %bb19
bb19: ; preds = %bb42, %bb10
br label %bb24.preheader
bb24.preheader: ; preds = %bb75, %bb19
store i32 0, i32* @g_993, align 4
br i1 %tmp6.i, label %bb24.preheader.split.us, label %bb24.preheader.split
bb24.preheader.split.us: ; preds = %bb24.preheader
br label %bb61.us
bb67.us.loopexit: ; preds = %bb65.us
br label %bb75
bb61.us: ; preds = %bb65.us, %bb24.preheader.split.us
br i1 false, label %bb65.us, label %bb81.loopexit
bb65.us: ; preds = %bb61.us
br i1 false, label %bb61.us, label %bb67.us.loopexit
bb24.preheader.split: ; preds = %bb24.preheader
br label %bb27
bb27: ; preds = %bb24.preheader.split
br i1 false, label %bb42, label %bb67
bb42: ; preds = %bb27
br label %bb19
bb67: ; preds = %bb27
br label %bb75
bb75: ; preds = %bb67, %bb67.us.loopexit
br i1 undef, label %bb24.preheader, label %bb84.loopexit
bb81.loopexit: ; preds = %bb61.us
br label %bb10
bb84.loopexit: ; preds = %bb75
ret void
}
attributes #0 = { "target-features"="+transactional-execution,+vector" }